Published on Web 01/30/2007
Redox-Switched Complexation/Decomplexation of K+ and Cs+ by Molecular Cyanometalate Boxes Julie L. Boyer, Maya Ramesh, Haijun Yao, Thomas B. Rauchfuss,* and Scott R. Wilson Contribution from the Department of Chemistry, UniVersity of Illinois at UrbanasChampaign, Urbana, Illinois 61801 Received June 30, 2006; E-mail:
[email protected] Abstract: The reaction of [N(PPh3)2][CpCo(CN)3] and [Cb*Co(NCMe)3]PF6 (Cb* ) C4Me4) in the presence of K+ afforded {K⊂[CpCo(CN)3]4[Cb*Co]4}PF6, [KCo8]PF6. IR, NMR, ESI-MS indicate that [KCo8]PF6 is a high-symmetry molecular box containing a potassium ion at its interior. The analogous heterometallic cage {K⊂[Cp*Rh(CN)3]4[Cb*Co]4}PF6 ([KRh4Co4]PF6) was prepared similarly via the condensation of K[Cp*Rh(CN)3] and [Cb*Co(NCMe)3]PF6. Crystallographic analysis confirmed the structure of [KCo8]PF6. The cyanide ligands are ordered, implying that no Co-CN bonds are broken upon cage formation and ion complexation. Eight Co-CN-Co edges of the box bow inward toward the encapsulated K+, and the remaining four µ-CN ligands bow outward. MeCN solutions of [KCo8]+ and [KRh4Co4]+ were found to undergo ion exchange with Cs+ to give [CsCo8]+ and [CsRh4Co4]+, both in quantitative yields. Labeling experiments involving [(MeC5H4)Co(CN)3]- demonstrated that Cs+-for-K+ ion exchange is accompanied by significant fragmentation. Ion exchange of NH4+ with [KCo8]+ proceeds to completion in THF solution, but in MeCN solution, the exclusive products were [Cb*Co(NCMe)3]PF6 and the poorly soluble salt NH4CpCo(CN)3. The lability of the NH4+-containing cage was also indicated by the rapid exchange of the acidic protons in [NH4Co8]+. Oxidation of [MCo8]+ with 4 equiv of FcPF6 produced paramagnetic (S ) 4/2) [Co8]4+, releasing Cs+ or K+. The oxidation-induced dissociation of M+ from the cages is chemically reversed by treatment of [Co8]4+ and CsOTf with 4 equiv of Cp2Co. Cation recognition by [Co8] and [Rh4Co4] cages was investigated. Electrochemical measurements indicated that E1/2(Cs+) - E1/2(K+) ∼ 0.08 V for [MCo8]+.
Introduction
Although three-dimensional coordination polymers based on cyanide have been known literally for centuries,1,2 the first molecular cyanometalate cage, [(OC)Pd(CN)Mn(C5H4Me)(CO)2]4, was reported only in 1990.3 Since 1998, several cyanometalate cages have been described. Such cages are typically prepared via condensation of facial tricyanometalates, such as [L3M(CN)3]z-, with facial tritopic Lewis acids (Scheme 1).4,5 Cyanometalate cages adopt diverse structures,6-9 but the most prevalent motif is the expanded cube or box comprised of eight (1) Dunbar, K. R.; Heintz, R. A. Prog. Inorg. Chem. 1997, 45, 283-391. (2) Sharpe, A. G. The Chemistry of Cyano Complexes of the Transition Metals; Academic Press: London, 1976. (3) Braunstein, P.; Oswald, B.; Tiripicchio, A.; Camellini, M. T. Angew. Chem., Int. Ed. Engl. 1990, 29, 1140-1143. (4) (a) Klausmeyer, K. K.; Rauchfuss, T. B.; Wilson, S. R. Angew. Chem., Int. Ed. 1998, 37, 1694-1696. (b) Heinrich, J. L.; Berseth, P. A.; Long, J. R. Chem. Commun. 1998, 1231-1232. (5) Schelter, E. J.; Prosvirin, A. V.; Reiff, W. M.; Dunbar, K. R. Angew. Chem., Int. Ed. 2004, 43, 4912-4915. (6) (a) Berseth, P. A.; Sokol, J. J.; Shores, M. P.; Heinrich, J. L.; Long, J. R. J. Am. Chem. Soc. 2000, 122, 9655-9662. (b) Lang, J.-P.; Xu, Q.-F.; Chen, Z.-N.; Abrahams, B. F. J. Am. Chem. Soc. 2003, 125, 12682-12683. (c) Lee, I. S.; Long, J. R. Dalton Trans. 2004, 3434-3436. (d) Sokol, J. J.; Shores, M. P.; Long, J. R. Inorg. Chem. 2002, 41, 3052-3054. (e) Sokol, J. J.; Shores, M. P.; Long, J. R. Angew. Chem., Int. Ed. 2001, 40, 236239. (f) Contakes, S. M.; Klausmeyer, K. K.; Milberg, R. M.; Wilson, S. R.; Rauchfuss, T. B. Organometallics 1998, 17, 3633-3635. (g) Kuhlman, M. L.; Yao, H.; Rauchfuss, T. B. Chem. Commun. 2004, 1370-1371. (h) Berlinguette, C. P.; Vaughn, D.; Canada-Vilalta, C.; Galan-Mascaros, J. R.; Dunbar, K. R. Angew. Chem., Int. Ed. 2003, 42, 1523-1526. 10.1021/ja0646545 CCC: $37.00 © 2007 American Chemical Society
Scheme 1
M-CN-M′ linkages with idealized Td symmetry.4,5,10-12 Cyanometalate frameworks have been discussed as components of electronic devices,13,14 sensors,15 molecular magnets,16,17 and (7) Contakes, S. M.; Rauchfuss, T. B. Angew. Chem., Int. Ed. 2000, 39, 19841986. (8) Contakes, S. M.; Rauchfuss, T. B. Chem. Commun. 2001, 553-554. (9) Contakes, S. M.; Kuhlman, M. L.; Ramesh, M.; Wilson, S. R.; Rauchfuss, T. B. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4889-4893. (10) Li, D.; Parkin, S.; Wang, G.; Yee, G. T.; Clerac, R.; Wernsdorfer, W.; Holmes, S. M. J. Am. Chem. Soc. 2006, 128, 4214-4215. (11) Klausmeyer, K. K.; Wilson, S. R.; Rauchfuss, T. B. J. Am. Chem. Soc. 1999, 121, 2705-2711. (12) Hsu, S. C. N.; Ramesh, M.; Espenson, J. H.; Rauchfuss, T. B. Angew. Chem., Int. Ed. 2003, 42, 2663-2666. (13) Vahrenkamp, H.; Geiss, A.; Richardson, G. N. J. Chem. Soc., Dalton Trans. 1997, 3643-3651. (14) Bignozzi, C. A.; Schoonover, J. R.; Scandola, F. Prog. Inorg. Chem. 1997, 44, 1-95. (15) Beauvais, L. G.; Shores, M. P.; Long, J. R. J. Am. Chem. Soc. 2000, 122, 2763-2772. (16) Beltran, L. M. C.; Long, J. R. Acc. Chem. Res. 2005, 38, 325-334. J. AM. CHEM. SOC. 2007, 129, 1931-1936
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sieves.11,12 The rigidity of cyanometalate frameworks should enhance the selectivity of their host-guest behavior. For example, the {[CpCo(CN)3]4[Cp*Ru]4} cage exhibits what appears to be the highest known preference for binding of Cs+ versus K+ (KCs/KK > 1012).12 This selectivity is conceptually relevant to the separation of radioactive 137Cs from solutions containing K+.1 The complementation of supramolecular systems with redox behavior is both well established and topical.18 Incorporation of redox-active components into cyanometalate boxes should allow the host-guest behavior of the box to be modified via changes in the redox poise of the host cage. In previous work on redox-active ion receptors, Plenio described a ferrocenemodified cryptand that exhibits an 80 mV difference when complexed to K+ versus Cs+.19 The E1/2 for a bis(calix[4]diquinone) sequestrant displays a 60 mV difference for its K+ versus Cs+ derivative, although the effect is largely obscured by the multielectron nature of the couple.20 Aside from possible sensor applications, a second motivation for the study of redoxactive sequestrants is that their affinities could be switched on and off electrochemically. One would expect that strongly held guests could be decomplexed from hosts by oxidation since cationic frameworks would bind cations more weakly than reduced derivatives. The cyanometalate cages are especially interesting because these structures envelop the guest ion, maximizing the intimacy of the host-guest interaction and, therefore, potentially maximizing the responsiveness of hostguest behavior to the redox state of the cage. In this report, we describe the use of the [Cb*Co]+ fragment (Cb* ) η4-C4Me4) for the construction of electro-active cyanometalate cages. The general utility of [Cb*Co(NCMe)3]+ was established by Herberich,21 who prepared this species in good yield starting from 2-butyne and Co2(CO)8. Crystallographic studies of [Cb*Co(NCMe)3]PF6 showed that the range of N-Co-N bond angles varies from 97.8 to 89.0, which is compatible with box-like cages.
Table 1. Selected Bond Distances (Å) and Angles (deg) for {K⊂[CpCo(CN)3]4[Cb*Co]4}PF6
Co1-C1 Co1-C2 Co1-C3 Co2-N1 Co2-N2 Co3-N3 K-C1 K-C2 K-C3 K-N1 K-N2 K-N3
1.8663(0.0035) 1.8566(0.0040) 1.8787(0.0036) 1.9607(0.0028) 1.9613(0.0032) 1.9679(0.0028) 3.5326(0.0039) 3.8406(0.0039) 3.4559(0.0039) 3.3825(0.0032) 3.8947(0.0034) 3.3025(0.0032)
C1-Co1-C2 C1-Co1-C3 C2-Co1-C3 N1-Co2-N2 N1-Co2-N3 N2-Co2-N3 Co1-C1tN1 Co1-C2tN2 Co1-C3tN3 Co2-N1tC1 Co2-N2tC2 Co2-N3tC3
92.40(0.15) 84.15(0.15) 93.11(0.15) 90.36(0.12) 95.85(0.12) 90.35(0.11) 172.73(0.32) 176.97(0.30) 174.13(0.31) 171.32(0.27) 163.02(0.27) 170.23(0.28)
Results and Discussion
Condensation Routes to Co8 and Co4Rh4 Cages. The reaction of equimolar amounts of [N(PPh3)2][CpCo(CN)3] and [Cb*Co(NCMe)3]PF6 gave insoluble, apparently nonmolecular products. When, however, the same reaction was conducted in the presence of K+, a deep purple solution resulted, from which we could isolate the salt {K⊂[CpCo(CN)3]4[Cb*Co]4}PF6, [KCo8]PF6. The IR spectrum of this salt featured a band for νCN at 2147 cm-1 versus 2121 cm-1 for K[CpCo(CN)3] in MeCN solution. ESI-MS confirmed the presence of [KCo8]+ (m/z ) 1515 amu). The 1H NMR spectrum confirmed the high symmetry of [KCo8]+, with singlets at δ 5.58 (Cp) and 0.96 (Cb*). Solutions of [KCo8]+ rapidly decompose in air to give an insoluble solid. The analogous heterometallic {K⊂[Cp*Rh(CN)3]4[Cb*Co]4}PF6 ([KRh4Co4]PF6) cage was prepared via the condensation (17) Holmes, S. M.; Girolami, G. S. J. Am. Chem. Soc. 1999, 121, 5593-5594. (18) (a) Boulas, P. L.; Gomezkaifer, M.; Echegoyen, L. Angew. Chem., Int. Ed. 1998, 37, 216-247. (b) Kaifer, A. E. Acc. Chem. Res. 1999, 32, 62-71. (c) Tucker, J. H. R.; Collinson, S. R. Chem. Soc. ReV. 2002, 31, 147-156. (d) Beer, P. D.; Gale, P. A.; Chen, G. Z. J. Chem. Soc., Dalton Trans. 1999, 1897-1910. (19) Plenio, H.; Aberle, C. Organometallics 1997, 16, 5950-5957. (20) Webber, P. R. A.; Beer, P. D.; Chen, G. Z.; Felix, V.; Drew, M. G. B. J. Am. Chem. Soc. 2003, 125, 5774-5785. 1932 J. AM. CHEM. SOC.
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Figure 1. The molecular structure of the metal cyanide core of {K⊂[CpCo(CN)3]4[Cb*Co]4}PF6 with the Cp and Cb* ligands omitted for clarity. Thermal ellipsoids are drawn at the 50% level. N ) blue, C ) gray, Co ) purple, K ) pink.
of K[Cp*Rh(CN)3] and [Cb*Co(NCMe)3]PF6. The formation of [KRh4Co4]+ was confirmed by 1H NMR and IR spectroscopies as well as ESI-MS. The IR spectrum of this salt featured a single νCN band at 2146 cm-1, and the 1H NMR spectrum confirmed the high symmetry. Crystallographic Study of [KCo8]PF6. The box adopts a structure typical for M8(CN)12 cages. The cyanide ligands are ordered, as revealed by the CpCo-C distances being on average 0.096 Å shorter than the Cb*Co-N distances. Further evidence that the cyanide ligands remain ordered is provided by the greater linearity of the average Co-CtN angle (174.6°) versus the average Co-NtC angle (168.2°) (see Table 1 and Figure (21) Butovskii, M. V.; Englert, U.; Fil’chikov, A. A.; Herberich, G. E.; Koelle, U.; Kudinov, A. R. Eur. J. Inorg. Chem. 2002, 2656-2663.
K+ and Cs+ Complexation/Decomplexation by Cyanometalate
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Scheme 2. Ion Exchange in [MCo8]+ Cages
1). The cyanide linkers deviating from linearity have been previously reported.5,7,8,22 Distortion from Td symmetry is due in part to the C-Co-C and N-Co-N angles, which range from 84.15 to 93.11° and 90.35 to 95.85°, respectively. Considering Cb* as a bidentate ligand, one might expect that the N-Co(I)-N angles would be significantly greater than 90°, but they are 90.35, 90.36, and 95.35°. For each [CpCo(CN)3]- subunit, two cyanide groups form short contacts and one forms a longer contact with the encapsulated cation. Thus, eight Co-CN-Co edges of the cube bow inward toward the encapsulated K+, and the remaining four µ-CN ligands bow outward. The four edges that bow outward display K-C and K-N distances of 3.841(34) and 3.893(39) Å, respectively, whereas the remaining eight edges of the cube display shorter K-C distances of 3.5326(34) and 3.4555(34) Å and K-N distances of 3.3827(29) and 3.3035(28) Å. The distortion of the [KCo8]+ cage due to nonlinearity of the cyanide bridges can be compared to other potassium-containing cages such as (NEt4)3{K⊂[Cp*Rh(CN)3]4[(CO)3Mo]4}.11 In this cubic cage the average Rh-CtN angle is 177.34° and the average MoNtC angle is 173.29°. Ion-Exchange Properties of Co8 Boxes. MeCN solutions of [KCo8]+ were found to react readily with Cs+ to give [CsCo8]+ (Scheme 2). The 133Cs NMR spectrum of [CsCo8]+ exhibits a singlet at δ 0.12, which is consistent with binding of Cs+ at the interior of a cyanometalate box.11 1H NMR spectra do not clearly distinguish binding of Cs+ versus K+: for Cb* ∆δ is 0.006 and for Cp ∆δ ) 0.008 ppm. ESI-MS confirmed the presence of [CsCo8]+ (m/z ) 1609 amu). No signal was observed for the [KCo8]+ starting material. The [CsRh4Co4]+ analogue was prepared similarly from [KRh4Co4]+ and CsOTf. ESI-MS confirmed the presence of [CsRh4Co4]+ (m/z ) 2065) with no peak corresponding to [KRh4Co4]+ observed. [CsCo8]+ could also be efficiently generated by the Cs+templated reaction of PPN[CpCo(CN)3] and [Cb*Co(NCMe)3]PF6. When this same reaction was conducted with a deficiency of [Cb*Co(NCMe)3]PF6, ESI-MS indicated that [CsCo8]+ was the major product; peaks corresponding to seven-vertex cages,9,23 such as Cs[CpCo(CN)3]4[Cb*Co]3, were not observed. Labeling experiments demonstrated that Cs+-for-K+ ion exchange proceeds via significant fragmentation of the cage (22) Richardson, G. N.; Brand, U.; Vahrenkamp, H. Inorg. Chem. 1999, 38, 3070-3079. (23) Kuhlman, M. L.; Rauchfuss, T. B. J. Am. Chem. Soc. 2003, 125, 1008410092.
Figure 2. Cyclic voltammograms of [KCo8]+ (top) and [CsCo8]+ (bottom) at scan rates of 0.2 V/s in a 0.1 M NBu4PF6/MeCN solution.
followed by reassembly (Scheme 2). The robustness of the MCo8 cages was first established by examining the quasidegenerate reaction involving MeCN solutions of [KCo8]+ and K[Cp′Co(CN)3] (Cp′ ) MeC5H4): after 5 h at room temperature,
